Effect of pH or Nutrient Availability
One of the greatest influence of pH on plant growth is through its effect on the nutrient availability. When base saturation is less than 100 per cent, an increase in pH is associated with an increase in the amount of calcium and magnesium in the soil solution, because calcium and magnesium are usually the dominant exchangeable bases. Many studies have shown that plant growth is promoted with increase in percentage of calcium in plants, which in turn is governed by the increasing pH or percentage base saturation. The general relationship between pH and availability of calcium and magnesium and other nutrients is shown in figure below.
Availability of molybdenum increases at higher pH (see figure above). At low pH, molybdenum forms insoluble compounds with iron and is rendered unavailable. Under these conditions, plants like cauliflower, clover and citrus will suffer from molybdenum deficiency but will show better perfonnance when soil pH is increased. Potassium availability is usually good in alkaline soils that reflect the limiting leaching and removal of exchangeable potassium. The availability or solubility of some plant nutrients decreases at higher pH. Iron and manganese which are generally deficient in calcareous soils are two good examples. Phosphorus and boron also tend to be unavairable in acid soils. On the other hand copper and zinc have reduced availabiliry in both highly acidic and alkaline soils. The overall nutrient availability is good at close to pH 6.5.
Another aspeq of soil chemistry not indicated in the figure above is that at extremes of pH, the balance among nutrients becomes unfavourable, and certain nutrients become excessively soluble as to be toxic. For example, at low pH Al, Fe, Mn, Zn and Cu are excessively released to become toxic. The H and OH ions themselves are directly injurious below pH 4 or above 9.